As others wrote, I'm HIGHLY skeptical that the FUSC is truly transferrable (aka option code SC01). MOST sellers who claim this are simply offering to give you their Tesla account & password with the car. If this is the case, Tesla could easily shut down the FUSC if/when they ever realize that the original owner no longer has the car (say you take it in for service... EVER.)
Also most people value this feature (FUSC) at about $5000 when buying a car with it. FSD is $8000 new, but there are lots of used cars with FSD, and it's not valued at anything close to new when pricing a used car. Usually (even in Tesla's own used inventory) it is given a value of exactly $0.00. So, you're talking a value of at most, $13k, closer to $5000, depending on how much you value FUSC - which again assumes it will actually transfer.
Also, does the car actually have the latest FSD software installed and running? If not, they likely have MCU1 (MCU2 was not available when SC01 was available) and you'll have to deal with upgrading the MCU in order to get functional FSD. There are many threads detailing how difficult this is to do if you don't want to pay the $2000-ish upgrade fee (which you shouldn't - but that's another story).
Also as others have said, you get just order a brand new stock Model S right now for $73k, and it's a better car in almost every single way than a 2017 P100D. You would lose out on FUSC and FSD, but you would have a full warranty, better battery, motors, seats, computers, displays, a rear display, better suspension, efficiency/range, charging speed, new paint, full glass roof (if the P100D has a full glass roof, it does not have SC01 FUSC).... that's just off the top of my head.
This is a no brainer. Walk Run away and laugh at the guy. If you have $70k to consider this car, just buy a new one for $3k more and forego the FSD and FUSC.